The earth is largely water with only a tiny fraction available for drinking or irrigation. The majority of the water is contained in our oceans and is too salty for human consumption. Most of the water currently used for drinking and irrigation is fresh water at less than half of 1% of the global water supply. A considerable number of people on earth lack clean drinking water, with contaminated drinking water involved in a large percentage of all human illness and disease including gastroenteritis, dysentery, cholera and other waterborne diseases which claim many human lives each year. Abundant, clean water can improve the lives of rural dwellers worldwide.
The water supply systems in the United States are under increasing strain with reoccurring drought and contamination. Water is being removed from underground reservoirs known as aquifers too fast to allow for rainwater to refill these resources. Moreover, purification efforts of ocean water are presently insufficient to provide an adequate supply of fresh water.
Problems of water scarcity are intensified by pollution of our fresh water supplies. In the United States, trihalomethane gases, known to cause cancer in laboratory animals, contaminate virtually all of our drinking water as a result of the chlorination process that city water systems use to prevent the spread of waterborne diseases. Trihalomethanes form when chlorine interacts with algae, microorganisms or other organic materials in the water. Other contaminants originate in the delivery system—lead from water pipes leach into our tapwater. Pollutants are also contaminating groundwater. Salt thrown on icy roadways has worked its ways into aquifers in New England, and wells are vulnerable to contamination from dumped toxic chemicals, including pesticides. Once groundwater is contaminated it stays contaminated for many, many years.
People have relied on distillation as a separation technique to purify water for thousands of years. Distillation is a process of evaporation and condensation which involves boiling the feed liquid, moving its vapors to a different location, and condensing the vapors to obtain pure water product. The portion of the feed liquid that does not boil off becomes concentrated. This concentrated liquid known as “blowdown,” carries impurities out. The problem with distillation is the extremely high amount of energy it takes to boil water. About 1200 BTU per gallon (1.5 kwh per liter) are required to heat the feed water from 60° F. (16° C.) to 212° F. (100° C.), its boiling point. After feed water reaches its boiling point, about 8000 BTU (2.3 kwh) of heat energy are required to convert a gallon of it to steam.
In the past, people have purified water for consumption using a home distillation apparatus known as a still. Conventional tapwater stills consist of a boiling chamber, a condensing chamber, and an electric heater. The heater boils the impure water. Steam travels to the condensing chamber and condenses, becoming distilled water. These stills remove solid pollutants that contaminate the drinking water. But such stills won't remove toxic gases or liquids, which bubble off with the escaping steam, contaminating the product water.
There remains an urgent need for an efficient, point of use device which is simple to manufacture and use for processing water to remove impurities.